1. Field of the Invention
The present invention relates generally to laser technology for photolithography, and, more particularly, to optimization of extreme ultraviolet (EUV) light production through correction of the focus of a laser beam used to produce the EUV light.
2. Description of the Prior Art
The semiconductor industry continues to develop lithographic technologies which are able to print ever-smaller integrated circuit dimensions. Extreme ultraviolet (EUV) light (also sometimes referred to as soft x-rays) is generally defined to be electromagnetic radiation having wavelengths of between 10 and 110 nanometers (nm). EUV lithography is generally considered to include EUV light at wavelengths in the range of 10-14 nm, and is used to produce extremely small features (e.g., sub-32 nm features) in substrates such as silicon wafers. These systems must be highly reliable and provide cost-effective throughput and reasonable process latitude.
Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state that has one or more elements (e.g., xenon, lithium, tin, indium, antimony, tellurium, aluminum, etc.) with one or more emission line(s) in the EUV range. In one such method, often termed laser produced plasma (LPP), the required plasma can be produced by irradiating a target, such as a droplet, stream or cluster of material having the desired spectral line-emitting element with a laser beam at an irradiation site.
The spectral line-emitting element may be in pure form or alloy form (e.g., an alloy that is a liquid at desired temperatures), or may be mixed or dispersed with another material such as a liquid. This target droplet is delivered to a desired irradiation site (e.g., a primary focal spot) and illuminated by a laser source within an LPP EUV source plasma chamber for plasma initiation and the generation of EUV light. It is necessary for the laser beam, such as from a high power CO2 laser source, to be focused by a focusing optic on a position through which the target droplet will pass and timed so as to intersect the target droplet when it passes through that position in order to hit the target droplet properly to obtain a good plasma, and thus, good EUV light.
Among possible issues affecting focus of the high power laser source on the target are collimation issues with the laser beam from the laser source, and aberrations introduced by the optical path including the focusing optic. This focusing optic is known as a final focus lens (FFL), and may be a lens.
The laser beam produced by the laser source should be collimated, that is, the light rays forming the beam are parallel. The focusing optic is commonly designed as an infinite conjugate, which assumes the laser beam received by the focusing optic is collimated. In practice, the beam from the laser source may be divergent, with the beam size increasing with distance, or convergent, with the beam size decreasing with distance. Collimation errors may also be introduced by other optical elements along the laser beam path. Deviations from a high degree of collimation in the laser beam thereby affect the focus of the laser beam on the target droplet, which can result in reduced laser power on the target droplet, and reduced EUV production.
The focusing optic focuses the laser beam on the target droplet. This focusing optic is known as a final focus lens (FFL). Focus errors in the final focus lens can spread the power of the laser source over a larger volume, reducing the power of the laser source delivered to the target droplet. Reduced laser source power delivered to the target droplet can result in a reduced EUV power production.
In the optical arts, a component known as a variable radius mirror (VRM) has been developed, for example by II-VI Infrared Incorporated. Such a variable radius mirror comprises a liquid filled cavity covered with a deformable reflective surface. By varying the pressure of the liquid in the cavity, the reflective surface is deformed, thereby changing the effective radius of the reflective surface. The effective radius of the variable radius mirror is thus determined by the pressure of the liquid in the cavity covered by the reflective surface.
For use with high power infrared (CO2) lasers, the reflective surface of the VRM is typically a thin copper sheet with optical coatings to increase reflectance at the operating wavelength. While the resulting reflectance is high, for example, 98% to 99%, when used with multi-kilowatt high power CO2 lasers, such reflectance values still result in significant energy transfer from the laser beam into the reflective surface, causing heating. To prevent thermal damage caused by overheating, the reflective surface must therefore be cooled. This has been accomplished by flowing the pressurized liquid through the cavity to thereby remove the generated heat from the reflective surface.
Testing has revealed that such a VRM is not suitable for use prior to the final focus lens in the optical system of a LPP EUV light source, as the fluid flow through the VRM generates perturbations in the reflective surface which cause instability in focus of the laser beam.
What is needed, therefore, is a way to correct the focus of the laser beam on the target in a LPP EUV system while avoiding instability in focus of the laser beam caused by VRM fluid flow.